The present invention relates to fiber optics, and more particularly to rare earth doped fiber optic amplifiers.
Wavelength division multiplexing (WDM) or dense WDM (DWDM) communication systems increase the transmission capacity of an optical fiber in a communication system by combining several optical signals having different wavelengths onto the optical fiber. Generally these systems include an optical amplifier with a pump laser that provides pump power and rare earth doped optical fiber. The optical signals are coupled to the rare earth doped fiber and the pump power and rare earth doped fiber amplify the optical signals. These amplifiers may also include input and output isolators and gain flattening filters.
To render these systems practical, the optical amplifiers must meet tight gain uniformity specifications over all channel wavelengths. Optical fibers often contain 80 channels at 80 wavelengths. The intrinsic gain spectrum of a rare earth doped fiber amplifier is highly non-uniform. Therefore, there are slight variations in the amount of amplification that is provided by the optical amplifier at the different wavelengths. The gain flattening filters flatten the gain profile of the channels after amplification.
The gain profile of the fiber amplifier is complicated by other factors as well. The gain profile of a rare earth doped optical fiber amplifier (OFA), such as an erbium doped fiber amplifier (EDFA), is determined by the average inversion level of the erbium ions in the erbium doped optical fiber. This inversion level is a function of the power level of the signal or signals to be amplified and the applied power levels of the laser pump sources. If the signal power is sufficiently lower than the applied pump power, the fiber will maintain close to 100% inversion and the signal gain and the amplifier gain profile will not appreciably change with changes in input signal powers. However, as signal power increases, the signal gain in the amplifier becomes limited by the availability of pump power for the fiber amplifier. In other words, the output signal power is limited by the available pump power and the signal input power levels.
As a result of these complex design considerations, rare earth doped fiber optical amplifiers are very expensive. The design and cost of optical switch fabrics is becoming more critical to optical system architectures. As the transparency of the optical network increases, the need for switching and variable optical add/drops (VOAD) will also increase. In other words, adding and dropping individual channels will become more common. The insertion loss from the switch fabric typically varies between 5 and 15 decibels (dB). The optical amplifiers that are associated with VOAD need to compensated for this insertion loss. However, the optical amplifiers described above will be too costly to implement or duplicate for individual channels. Therefore, optical amplifiers that can amplify individual channels and that are economical to implement are needed.
An optical amplifier according to the invention includes a plurality of optical paths each carrying an optical signal and each including active optical fiber. A shared pump laser is coupled to the plurality of optical paths and provides pump power to the plurality of optical paths to individually amplify the optical signals.
In other features of the invention, the plurality of optical paths include input and output optical isolators and a coupler for coupling the pump power to the optical path. The active optical fiber is doped with an implant selected from the group of rare earth metals, erbium, ytterbium, and both ytterbium and erbium. A variable attenuator is connected between the pump laser and the coupler of at least one of the plurality of optical paths or adjacent to the output isolator of one of the optical paths.
In another aspect of the invention, an optical amplifier includes an optical path including a plurality of active optical fiber sections. A laser pump is coupled to the optical path and provides pump power on the optical path. A plurality of optical signals are serially coupled to the optical path at the active optical fiber sections, amplified, and decoupled from the optical path.
In other features of the invention, the optical paths include input and output optical isolators and a coupler for coupling the pump power to the optical path. The active optical fiber is doped with an implant selected from the group of rare earth metals, erbium, ytterbium, and both erbium and ytterbium.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.